1. Field of the Invention
The present invention relates generally to methods and systems for non-destructive testing and inspection of pipes, tubes, and other longitudinal cylindrical structures. The present invention relates more specifically to methods and systems for accurately positioning long-range torsional guided-wave inspection sensors on waterwall tubes in boiler structures and the like.
2. Description of the Related Art
Most fossil fuel based power generating systems utilize the heat released from burning the fuel to convert water to high pressure steam that is then used to turn a steam turbine connected to an electrical generator. One of the most efficient ways of heating water to convert it to steam involves the use of a waterwall boiler wherein the fuel is burned within a confined furnace space defined by walls made up of an array of water tubes. As relatively cool water passes through the tubes it absorbs the heat from the burning fossil fuel and eventually exits the array of tubes as steam.
The most common cause of forced outage of fossil fuel powered generating units is boiler tube failure. The majority of these failures occur within the furnace waterwall tubes. Various damage mechanisms are known to occur that lead to tube failures through wall loss and cracking. Most of the damage to waterwall tubes occurs on the fire side of the tube by way of wall thinning due to corrosion and/or erosion that results from the furnace exposure and the presence of corrosive gases in the fuel burning process.
To prevent or reduce such boiler tube failures during plant operation, the tubes within such waterwalls are inspected nondestructively during normal outages, typically using ultrasonic wall-thickness measurements. Since the boiler waterwall is a very large structure and ultrasonic wall-thickness measurements are time consuming, thickness measurements are typically taken at several points along the height of the wall. Maintenance decisions regarding the overall condition of the boiler are therefore made based upon a statistical analysis of the very limited measurement data. As a result, the reliability of the current decision making process is less than desired and carries a substantial risk of error. The reliability of the boiler would be significantly improved if the condition of the boiler were determined based on the inspection results of a large portion of the boiler wall, if not the total boiler wall, rather than results from very limited measurement points.
Long-range guided-wave inspection technology is an emerging technology that has the capability of quickly surveying a large volume of a structure for defects and providing comprehensive condition information on the integrity of the structure. Using relatively low frequency (typically under 200 kHz) guided-waves in the pulse echo testing mode, this technology performs a 100% volumetric examination of a large area of a structure and detects and locates internal and external defects in the area around a given test position. In exposed, single tube pipelines, for example, a test range of more than five hundred feet can be achieved in one direction for detecting 2% to 3% defects from a given test position. Percent in such examples refers to the circumferential cross-sectional area of the defect relative to the total pipe or tube cross-section. The guided-wave inspection technology, including the magnetostrictive sensor technology developed at Southwest Research Institute in San Antonio, Tex., is now widely used for testing piping networks in processing plants such as refineries, chemical plants, and power generating stations. The preferred guided-wave mode for pipe or tube inspection is torsional (T) wave mode.
For generalized piping inspection, guided-wave probes that encircle the entire pipe circumference are presently in use. To install a guided-wave probe for piping inspection, the basic systems and methodologies require full access around the pipe circumference with about three to five inches of spacing. When access is limited to only a portion of the piping circumference, the long-range guided-wave inspection method is difficult to apply. Waterwall tubes constructed in boiler furnaces present just such an environment where access to the full circumference of an individual tube is not possible. A typical firewall tube might be constructed from metal components that appear on the outside surface to be an array of closely spaced parallel pipes when; in fact, they most often comprise a unitary structure where no space exists between the pipes forming the wall. An example of a cross-section of a typical firewall tube structure can be seen in FIG. 1 of the present application.
Some efforts have been made in the past to provide sensor structures and methodologies for their use directed to waterwall boiler tubes. As indicated above, such inspections are typically carried out using ultrasonic sensors and methodologies, although the limited range of such sensors requires sampling techniques to be utilized during testing. Other efforts in the field have included those described in the following U.S. patents:
U.S. Pat. No. 5,526,691 issued to Latimer et al. on Jun. 18, 1996 entitled Detection of Corrosion Fatigue Cracks in Membrane Boiler Tubes describes a method for detecting defects and anomalies in boiler tubes arranged in a panel and associated with a waterwall. The system utilizes at least one EMAT (Electromagnetic Acoustic Transducer) coil that generates ultrasonic shear waves at a predetermined beam angle. The method is alleged to provide a better signal-to-noise ratio than conventional ultrasonic techniques and to further eliminate the need for a couplant between the sensor and the boiler tube.
U.S. Pat. No. 6,125,703 issued to Mac Lauchlan et al. on Oct. 3, 2000 entitled Detection of Corrosion Fatigue in Boiler Tubes Using a Spike EMAT Pulsar describes a further EMAT based method for detecting damage in ferromagnetic boiler tube structures using a pair of EMAT coils adjacent the work piece at a non-zero angle with respect to one another. A spike pulse is applied to one of the EMAT coils to generate a horizontally polarized shear wave which is reflected by flaws and defects in the work piece and subsequently received by the second EMAT coil.
U.S. Pat. No. 6,497,151 issued to Watts et al. on Dec. 24, 2002 entitled Non-Destructive Testing Method and Apparatus to Determine Micro Structure of Ferrous Metal Objects describes a method and apparatus for non-destructively investigating structures such as cast iron pipes. A sonic wave in induced in the object and a sensor assembly captures the acoustic signal from the object. The data analysis system calculates the energy of the acoustic wave or calculates the time from its initial induction to determine a nodularity measurement of the metal object.
U.S. Pat. No. 5,359,898 issued to Latimer on Nov. 1, 1994 entitled Hydrogen Damage Confirmation with EMATs describes a method and apparatus for confirming hydrogen damage in boiler tubes that comprises a pair of electromagnetic acoustic transducer coils (EMATS) that are mounted for movement toward and away from each other. An electromagnet produces pulses that generate acoustic waves across a chord and within the wall thickness of the boiler tube. The sensor is designed to adapt to boiler tubes of different diameters by mounting the transducer coils in such a manner that the coils can be pressed against the outer surface of the tubes. In concert, the angle of the acoustic beam between the coils is adjusted by changing the frequency of energy applied to the coils.
In general, the prior efforts in the field have been directed to the use of acoustic sensors or EMAT sensors. Very little effort has been made to create sensor structures appropriate for directing long-range guided-waves into such pipe walls, primarily because of the inability to fully encircle the circumference of the individual pipes. That is, none of the previous efforts that utilize partial circumferential orientation have provided suitable sensor adherence structures for long-range guided-wave inspection purposes. No sensor structures have been designed that can take advantage of the volumetric inspection capabilities of long-range guided-waves where access to the entire circumference of the pipe or tube is restricted. EMAT sensors, such as described in the above U.S. patents, are limited in that they fail to achieve the volumetric inspection capabilities of long-range guided-waves.
It would be desirable, therefore, to have a sensor structure and a method for its implementation, that overcomes many of the problems of existing sensor structures and the requirement that they either fully encircle the pipe or tube under inspection or that they utilize only local point inspection techniques such as ultrasonics or EMAT technologies.
In the present invention, systems and methods for inspecting the fire side of waterwall tubes in boilers, wherein limited access to the entire circumference of the tube is found, are described. The systems and methods are built upon existing magnetostrictive sensor (MsS) methods and devices, particularly the thin magnetostrictive strip approach (described in U.S. Pat. No. 6,396,262, entitled Method and Apparatus for Short Term Inspection or Long Term Structural Health Monitoring; U.S. Pat. No. 6,429,650, entitled Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes and Tubes; and U.S. Pat. No. 6,917,196, also entitled Method and Apparatus Generating and Detecting Torsional Wave Inspection of Pipes and Tubes, the disclosures of which are each incorporated herein in their entirety by reference) and a flat coil-type plate magnetostrictive sensor (MsS) (described in U.S. Pat. No. 6,294,912, entitled Method and Apparatus for Non-Destructive Inspection of Plate Type Ferromagnetic Structures Using Magnetostrictive Techniques, the disclosure of which is incorporated herein in its entirety by reference), the structures of which are held in place on the waterwall tubes under the influence of an activatable electromagnetic circuit.